Laser-based treatment of tissue is used for a variety of applications, such as hair removal, skin rejuvenation, wrinkle treatment, acne treatment, treatment of vascular lesions (e.g., spider veins, diffuse redness, etc.), treatment of cellulite, treatment of pigmented legions (e.g., age spots, sun spots, moles, etc.), tattoo removal, and various other treatments. Such treatments generally include delivering laser radiation to an area of tissue on a person's body, e.g., the skin or internal tissue, to treat the tissue in a photochemical, photobiological, thermal, or other manner, which can be ablative or non-ablative, among other properties, depending on the particular application.
Laser-based treatment devices may include any suitable type of laser, e.g., laser diode, fiber laser, VCSEL (Vertical Cavity Surface Emitting Laser), LED, etc. A device may include a single laser or multiple lasers, e.g., a laser diode bar including multiple distinct emitters arranged in a row, or multiple fiber lasers arranged in a row or array.
Diode lasers are particularly suitable for certain treatments and devices for providing such treatments. For example, diode lasers are compact, as they are typically built on one chip that contains all necessary components. Further, diode lasers typically provide an efficiency of up to 50%, which enables them to be driven by low electrical power compared to certain other lasers. Further, diode lasers allow direct excitation with small electric currents, such that conventional transistor based circuits can be used to power the laser.
Other characteristics of diode lasers include high temperature sensitivity/tunability, and a highly divergent beam compared to certain other lasers. Diode lasers typically emit a beam having an axis-asymmetric profile in a plane transverse to the optical axis of the laser. In particular, the emitted beam diverges significantly faster in a first axis (referred to as the “fast axis”) than in an orthogonal second axis (referred to as the “slow axis”). In contrast, other types of lasers, e.g., fiber lasers, typically emit a beam having an axis-symmetric profile in the transverse plane.
Laser-based treatment devices include larger-scale devices typically operated by a physician or other professional in a clinic or other office, as well as hand-held devices for home-use, allowing users to provide treatment to themselves. Some hand-held laser-based treatment devices are battery powered, e.g., using a Li ion battery cell (or multiple cells). Such battery-powered devices may be recharged between use, e.g., by plugging into an A/C wall outlet, either directly or by docking in a docking unit plugged into the wall.
Laser-based treatment devices are typically high current devices, in order to drive the laser(s) to generate the desired laser radiation. Battery-powered laser-based treatment devices include a control system to apply a relatively high current, e.g., 30-120 A from the battery to the laser(s) to generate the desired laser radiation for delivery to the skin. Battery-powered laser-based treatment devices often use a linear constant-current control system, with analog control.
FIG. 1 shows an example linear control system for providing constant-current from a battery to a laser diode LD. As shown, a continually varying voltage is generated by an analog circuit, which may include one or more op-amps, resistors, and/or other electronic components, which applies a gate voltage to a control FET (field effect transistor), which voltage is between the voltage that turns the FET fully OFF, Vg(off), and the voltage that turns the FET fully ON, Vg(on). In this way, the FET acts as a voltage-controlled resistor that dissipates significant power as it controls the current flowing through the laser diode LD and the rest of the circuit. The analog control circuit typically requires a number of parts, which may be expensive and have accuracy requirements and calibration requirements.
The high power dissipation of the control FET generates heat on the circuit board, which may negatively affect other parts of the circuit, and may cause reliability issues. Such unwanted heat may be relatively difficult and expensive to remove from the device. In the example shown in FIG. 1, an analog current-sense amp measures the current, and this analog voltage is used by the control system, and also read into a CPU. This analog current sensing is needed for the analog linear control system to provide a closed-loop feedback circuit that operates on a short timescale, e.g., as compared with the duration of a laser pulse.